The present invention relates to the removal of compounds from petroleum refinery streams which have been determined to foul process equipment. More specifically, it relates to a process for separating large, stable polycyclic aromatic compounds which form during the hydrocracking process and which foul process equipment by scaling which results in plugging flow in and around such equipment.
Petroleum refinery hydrocracking processes are well known and developed. Such processes upgrade mixtures of hydrocarbons to supply more valuable product streams.
Hydrocracking is a high severity hydrotreating operation in which high molecular weight compounds are cracked to lower boiling materials. Severity is increased by operating with increasingly acidic catalysts and possibly at higher temperatures and longer contact times than in hydrotreating. Increased hydrogen pressure controls deposits and catalyst fouling. Unlike thermal or catalytic cracking, hydrocracking decreases the molecular weight of aromatic compounds and fills a specific need for processing streams high in aromatic material, such as cycle stocks from catalytic or thermal cracking, coker products, or coal liquids. For example, catalytic cycle stock can be cracked to a naphtha fraction that is an excellent feed for catalytic reforming to make premium-octane gasoline or petrochemical aromatic material
Hydrocracking is used extensively on distillate stocks. The hydrocracking process is applied to refinery stocks for premium-quality kerosene, diesel and jet fuels. The light products from hydrocracking are also rich in isobutane, an important raw material for alkylation.
Hydrocracking is of increasing importance in view of the trend to heavier crudes and the need for processing synthetic crudes.
As demand for distillate fuels increased, refiners installed hydrocrackers to convert Vacuum Gas Oil (VGO) to jet and diesel. Catalysts were developed that exhibited excellent distillate selectivity, high conversion activity and stability for heavier feedstocks.
A trend in recent years in the push for higher yields of liquid products from hydrocracking units has been the use of longer life catalysts having an increasing amount of molecular sieve. A well known class of catalysts with a higher degree of molecular sieve are the "zeolite" type catalysts.
One result of the zeolitic catalyst in hydrocracking reactors is the formation of compounds in a class known as polycyclic aromatic compounds, or alternatively "polynuclear aromatics", or "PNA". Additionally, these polycyclic aromatic compounds were known to contribute to catalyst fouling and coking. The formation of polycyclic aromatic compounds has been found to increase during the catalyst run as hydrocracker temperatures increase.
In recent times, as the worldwide supply of light, sweet crude oil for refinery feedstock has become more scarce, there has been a significant trend toward conversion of higher boiling compounds to lower boiling ones. This "bottom of the barrel" or "hard processing" has increased potential downstream fouling problems by tending to create even greater quantities of heavier, converted cyclic compounds, such as polycyclic aromatics, in the initial stages of the refining process. The addition such process units as residual desulfurization units, makes the need for an economic solution to the fouling problem even more desirable.
In addition to high conversion distillate production, another trend in the 1980's has been to send unconverted fractionator bottoms from the hydrocracker to units such as FCC units, ethylene crackers and lube plants which benefit from highly paraffinic feedstocks. The fractionator bottoms material is desulfurized, denitrified and highly saturated during its residence time in the hydrocracker. The polycyclic aromatic compounds formed during the hydrocracking process, however, are quite undesirable in these other process units.
Extinction recycle hydrocrackers suffer from equipment fouling and plugging in the cooler portions of the process due to precipitation of certain polycyclic aromatic compounds.
U.S. Pat. No. 3,619,407 issued on Nov. 9, 1971 to Hendricks et al. describes one hydrocracking catalyst for use in a hydrocracking process, and is further relevant in describing certain aspects of the problem which is addressed by the present invention. The reference discloses the problem of the formation of polycyclic aromatic compounds which are identified in the reference as being "benzocorenene". The reference describes the known tendency for such compounds to "plate out" onto cooler downstream equipment such as heat exchanger surfaces. The claimed solution described in the reference is the withdrawal or "bleeding" of a significant portion of the hydrocracker effluent from the hydrocracking zone to a lower value stream such as fuel oil, in order to reduce the concentration of polycyclic aromatics existing in such effluent.
U.S. Pat. No. 4,447,315 issued on May 8, 1984 to Lamb et al. discloses a process scheme for reducing the concentration of certain polycyclic aromatic compounds, referred to in U.S. Pat. No. 4,447,315 as "PNA or benzocorenenes" in a hydrocracking process by separating hydrocracker effluent in a fractionator, and contacting fractionator bottoms in an adsorption unit with an adsorbent which selectively retains the "PNA compounds" described in U.S. Pat. No. 4,447,315, and recycling the fractionator bottoms back to the hydrocracking reactor.
U.S. Pat. No. 5,007,998, issued Apr. 16, 1991 to Gruia teaches a process for minimizing "11+ ring heavy PNA compounds" by hydrogenating a portion of the unconverted bottoms from a hydrocracking process in a separate reactor containing a zeolite catalyst.
In a technical paper by Sullivan et. al. entitled "Molecular Transformations in Hydrotreating and Hydrocracking", Journal of Energy and Fuels, Vol.3 p.603 (1989), which is fully incorporated by reference herein, assumptions about the route to the formation of large polycyclic aromatic compounds are discussed.
An effective and economical process for the removal of stable polycyclic aromatic compounds is much desired as a means of reducing fouling of refinery process equipment and catalyst coking.
A process for the identification and removal of stable polycyclic aromatic dimers found to foul equipment in a hydrocracking process, which process minimizes wastage of valuable streams and requires minimum capital investment and operating expense is much desired.
The present invention achieves the above desired outcomes without the shortcomings of the above processes.
Co-pending Application No. 567,427, assigned to the assignee of the present invention discloses a process for the selective precipitation and separation of stable polycyclic aromatic dimers present in hydrocracker effluent.